Latest revision as of 00:00, 21 December 2013

The goal of this project is to build a CNC router that can mill regular PCBs with non-SMD IC components, MDF, stryrofoam HDPE and nylon. The budget is around 1000 euro.
We ended up going for the MiniCnc, made at TimeLab Gent

the CNC router

The minimal distance between tracks when milling PCBs is about 0.2 mm, according to the people at timelab Ghent.
The maximal range is (X:173mm, Y:179mm).

At the moment the base plate is not parralel to the X,Y plane. There seems to be some glue residue on the base plate which we should sand off.

At the moment the accuracy is limited by the dremel, and the runout on its spindle. This can be seen when milling a point with a small diameter endmill,
instead of a small hole, a tiny circle will be milled. Probably the accuracy can be increased with a better spindle (with ball bearing) like the KRESS 800 FME

We added a LED to the carriage to illuminate the milling area.

Software

The CNC router controlled by sending it gcode. The sending of the gcode is done with replicatorG.
Multiple programs can be used to generate gcode.

ReplicatorG

ReplicatorG will report about a feedrate of zero, this is normal acording to https://groups.google.com/group/ultimaker/browse_thread/thread/6892f2b14d11b361 and is caused by our CNC lacking a feedback mechanism.
We are using replicatorG version 25.
if starting replicatorG normally it will not be able to control /dev/ttyUSB0 to talk to our CNC. So you either have to start replicatorG as root or run

sudo chmod 777 /dev/ttyUSB*

Cad.py

Cadpy can transform bitmaps to gcode. Beware that multiple versions of this tool exist, not all of which do work with our machine.

The last days we've discovered some problems when milling PCB's. The accuracy of the CNC router seems to be pretty good, however it seems that our current milling bits are not suitable to perform the job.
The ball point milling bit gives smooth tracks without sharp edges, but the milling width is heavily influenced when the PCB surface isn't flat.
The other fine milling bit could not deliver smooth tracks due to sharp edges.

Milling PCB's produces fine dust. A wooden box was made to prevent breathing in small copper particles.

Find a better way to fix the PCB's. The best way seems to be using double sided tape, egally applied to the bottom PCB surface. Then the PCB is attached to the flattened (by milling) MDF surface. The tape will help in straighten the PCB surface by pulling it against the MDF surface.

Buy strong double sided tape (not very thick)

Make a flat bottom surface. Please notice that 'flat' is relative to the CNC machine & milling spindle (Dremel). So the best way to do this is placing the CNC router on a new MDF panel (18mm minimum) and mill about 1mm of the surface away. Then we are sure that the relative height/depth is the same for each possible coordinate. We need the following:

I loaded the same test shape that we used before for testing PCB milling

Some parts on the PCB are milled deep enough, others are not

This can be solved by milling a square in the MDF bottom plate, so that the whole surface is equal in depth related to the CNC machine

With the 15° 0.1mm bit we should be able to increase the depth easily

Edges are very smooth (best so far)

Extrusion moulding

A nice guide made by a guy who makes robotics components in his garage. He mills out the parts, makes a silicone mold and then casts the parts in PU. Buildlog: buildlog, part 1 and part 2 of the guide.

ToDo

replace the plastic guiding bearings with some better metal bearings. One of the plastic ones has the tendency to stay behind

make .g (the extension for gcode) an accepted extension on the wiki so we can upload it here.

Create an emergency stop button. While there is a stop button in replicatorG, this doesn't immediately stop the CNC, which will first execute some cached instructions.

while the endmill is less than 1mm, it's path seems to be about 2mm to the naked eye. This greatly decreases our accuracy.